Silicon (Si) and a silicon oxide represented by SiOX have higher capacity per unit volume as compared to carbon materials such as graphite and therefore are under investigation for applications in negative electrode active materials. In particular, SiOX is expected to be put into practical use early because SiOX has a less volume expansion coefficient as compared to Si when SiOX stores Li+ during charge. For example, Patent Literature 1 proposes a nonaqueous electrolyte secondary battery containing a negative electrode active material prepared by mixing SiOX with graphite.